Different sizes for different depths. The thicker cables are actually for shallow depths, where ship anchors and fishermen's nets can cause damage.

The cable-laying beauty herself, the Ile de Batz. She is one of three cable ships in Alcatel-Lucent's arsenal. Each ship can lay roughly 120 miles of undersea cable per day, at a depth of up to eight kilometers.

This is an image of one of the many repeaters that Alcatel-Lucent ships lay down at points along an undersea cable to boost the signal. This particular example is two years old and boasts a throughput of approximately 1. terabits/second.

When laying cable in shallow seas, a plow is used to bury the cable three meters into the sea floor. The plow works in waters up to 1.5km deep. [CNET]